Hackers on Saturday have managed to tale down at least three of NATO's web sites, following the growing tensions over Crimea.

The cyber attack was performed by a group calling itself "cyber berkut". According to the group's page (www.cyber-berkut.org), the attack had been carried out by patriotic Ukrainians angry over what they saw as NATO interference in their country.

"We [KiberBerkut] will not allow the presence of NATO and the occupation of our territory," the attackers say on their web page.

The main NATO website www.NATO.int, the http://ccdcoe.org/, and the http://nato-pa.int/ web addresses were not accesbible by the time of this publication.

The alliance confirmed the distributed denial-of-service (DDoS) attacks to Reuters, adding that none of its essential systems were compromised.